1. Field of the Invention
The present invention relates generally to a catalyst device and a combustion apparatus provided therewith, particularly to a catalyst device for purifying or deodorizing domestic or industry exhaust gas and also to the combustion apparatus provided with such a catalyst device.
2. Description of Related Art
Conventionally, the catalyst device generally comprises a honeycomb structure substrate made of ceramics such as cordierite or mullite, which is coated by an activated alumina called a "washcoat" on which surface of the catalyst device is then dispersed a catalyst metal powder. Further, a corrugated alumina/silica fiber or thin metal structure provided with the catalyst metal on the surface thereof has been sometimes used.
However, the honeycomb structure made of ceramics is beset by the following problems: 1) it takes a relatively long time to preheat the catalyst device up to a sufficient temperature because of its large heat capacity; 2) it is difficult to transfer a reactive heat from an upstream part of the catalyst device to a downstream thereof because of its poor heat conductivity in use of combustion catalyst; and 3) the corrugated structure made of alumina/silica fiber is easily broken because of its brittleness. On the other hand, in the case of a thin metal structure, although it is not easily broken, it also takes a long time to preheat the catalyst device because its good heat conductivity causes heat dispersion. Further, in the case of the alumina/silica fibers mixed with metal fibers, it is difficult for it to resist high temperatures of about 1000.degree. C. and it is also difficult to prepare a uniform structure in mass production due to drift tendencies of metal fibers.
In the catalyst combustion, a liquid fuel, such as petrol, premixed with air is suddenly oxidized on the catalyst layer to generate reaction heat and gas such as carbon dioxide and steam. The catalyst reaction usually occurs only at the upstream side near the catalyst device during the first stage, and a continuous high temperature under an oxidizing condition promotes deterioration of the upstream part. Accordingly, in order to prolong the life of the catalyst layer it is necessary to make a temperature difference between the upstream and the downstream parts as small as possible. Further, it is required to make the preheating time for activating the catalyst as short as possible, and thus it is necessary to make the heat capacity of the catalyst device as small as possible.